What if wiring for power supply was done to your house after building it? Or say how comfortable would you be if pipes for water supply were to be fitted after you moved into your house?
It would be a disaster (period). I am sure that even the sloppiest architect in this world would plan for power and electricity supply well before building the house. Similarly SEO (Search Engine Optimization) should be an integral part of your website structure and it should be accounted for in every part/page of the website you are about to build.
Leading open source CMS and ecommerce systems have user contributed SEO extensions. For example All in One SEO Pack for WordPress, SEO-Generator extension for Joomla automatically generate title and meta tags that is critical to even get your pages indexed in search engines.
So plan for SEO well before you start building your website. Advantages? I need not even mention that.